In some types of radio frequency (“RF”) transmitters, the RF output signal of an RF amplifier is required to closely match one of several predetermined amplitude envelopes. For example, RF transmitters exist where the transmitter output consists of continuous wave (“CW”) or phase modulated RF pulses that are required to closely match one of several predetermined amplitude envelopes. One such transmit mode is known as Tactical Air Command And Navigation (“TACAN”) involving amplitude modulation. Another is Time Domain Multiple Access (“TDMA”) involving phase modulation. Unfortunately, RF transmitters and their components are often subject to relatively severe temperature stresses, which can make it more difficult to match the desired waveform envelope.
In addition, transmitter frequency range, output power, size and efficiency requirements associated with, for example TACAN/TDMA, have dictated the use of highly nonlinear (typically ‘class C’) devices in the RF power amplifiers. To meet the envelope accuracy requirements of these nonlinear devices, closed loop control has been used to reduce the difference between the actual RF amplifier output waveform and a desired (as set by industry standards) waveform. However, during the interval between transmitted pulses, no RF energy is allowed to be transmitted; therefore a closed loop condition cannot exist during this interval.
A single control loop, operating under this constraint, having sufficient gain and bandwidth to meet the accuracy requirements over some portions of the output envelope, will likely be unstable over other portions of the output envelope. The instability is due primarily to the nonlinearities in the controlled element (the actual RF amplifier chain) and the absence of a valid closed loop condition in the time interval between RF pulses.
The control of transmitted RF pulses by single, analog closed loops is well known (e.g., Philips part # PCF5708, “Power Amplifier Controller for GSM and PCN Systems”, the data sheet for an IC controller). However, the waveform rise and fall times for GSM are approximately 30 μsec versus 3.5 μsec for Joint Tactical Radio System (“JTRS”) and TACAN and 0.25 μsec for TDMA, therefore the previous solutions do not provide the required control bandwidth.
Existing controllers have also generally been used with RF amplifiers that exhibit an approximately linear control transfer function (e.g., gate voltage control in a GaAs FET module). By contrast, due to the low gain of the devices required for JTRS-compatible applications, many stages are needed to achieve the required gain and output power. To meet efficiency requirements, each stage is normally operated under ‘class B’ or ‘class C’ conditions, and the RF input to RF output amplitude transfer function of the cascaded or series connection of multiple stages is therefore highly nonlinear.
Conventionally, the closed loop output distortion (the deviation of the RF output envelope from an ideal shape) is minimized by increasing both the closed loop gain and bandwidth up to the stability limit imposed by hardware delays and phase shifts within the closed loop. By contrast, the maximum allowed spurious, or modulation dependent, RF sideband output levels of a JTRS/TACAN transmitter are relatively low, and can be seriously degraded (increased) by excessive gain in the waveform control loop. In practice, therefore, the selection of the optimum loop gain for a given set of conditions is a compromise between the conflicting requirements of waveform distortion and minimum sideband noise. This optimum point varies with temperature, required power level and operating frequency.
Thus, there exists a need for an RF envelope amplitude controller that is configured to achieve RF output envelope control in applications having high frequencies, and/or nonlinearities, such as TACAN/TDMA applications. There also exists a need for a flexible RF envelope amplitude controller that can readily adjust for temperature, operating power mode, operating frequency, and waveform changes.